Hot-pressed member

ABSTRACT

A hot-pressed member includes a steel sheet, a Ni-diffusion region present in a surface layer of the steel sheet, and an intermetallic compound layer and a ZnO layer which are provided in order on the Ni-diffusion region, the intermetallic compound layer corresponding to a γ phase present in a phase equilibrium diagram of a Zn—Ni alloy, wherein a spontaneous immersion potential indicated in a 0.5 M NaCl aqueous air-saturated solution at 25° C.±5° C. is −600 to −360 mV based on a standard hydrogen electrode.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. patentapplication Ser. No. 13/504,755, filed Aug. 9, 2012, which is the U.S.National Phase application of PCT International Application No.PCT/JP2010/069643, filed Oct. 28, 2010, and claims priority to JapanesePatent Application Nos. 2009-247384, filed Oct. 28, 2009, 2010-102849,filed Apr. 28, 2010, and 2010-218094, filed Sep. 29, 2010, thedisclosures of each of which are incorporated herein by reference intheir entireties for all purposes. This application is also acontinuation application of U.S. patent application Ser. No. 14/093,995,filed Dec. 2, 2013, and U.S. patent application Ser. No. 14/094,003,filed Dec. 2, 2013, the disclosures of each of which are incorporatedherein by reference in their entireties for all purposes.

FIELD OF THE INVENTION

The present invention relates to a hot-pressed member produced bypressing a heated steel sheet, and particularly to a hot-pressed memberused for underbody parts and car body structures of automobiles and amethod for producing the same.

BACKGROUND OF THE INVENTION

Many underbody parts and body structural members of automobiles havebeen produced by pressing steel sheets having predetermined strength.From the viewpoint of global environment conservation, weight lighteningof automobile car bodies has recently been desired eagerly, and theeffort to decrease the thickness of the steel sheet used bystrengthening the steel sheet has been continued. However, the pressingworkability decreases with strengthening of steel sheets, and thus it isoften difficult to process steel sheets into desired member shapes.

Therefore, Patent Literature 1 proposes a processing technique referredto as “hot-pressing” which enables both easy working and strengtheningby quenching and processing a heated steel sheet at the same time usinga mold including a die and a punch. However, in the hot-pressing, thesteel sheet is heated to a high temperature of about 950° C. before thehot-pressing, and thus scales (Fe oxides) are produced on a surface ofthe steel sheet and are separated during the hot-pressing, therebycausing the problem of damaging the mold or damaging a surface of amember after the hot-pressing. In addition, the scales remaining on asurface of the member causes a poor appearance, a decrease in coatingadhesion, or a decrease in corrosion resistance after coating.Therefore, the scales on a surface of the member are generally removedby a treatment such as pickling or shot blasting, but this complicatesthe production process and decreases productivity.

From this viewpoint, there has been demand for a hot-pressing techniquecapable of suppressing the formation of scales during heating beforehot-pressing and improving coating adhesion and corrosion resistanceafter coating of a member after the hot-pressing, and a steel sheethaving a film such as a coating layer provided on a surface thereof, anda hot-pressing method using the steel sheet have been proposed.

For example, Patent Literature 2 discloses a coated steel sheet coatedwith Al or an Al alloy. It is described that by using the coated steelsheet, decarburization and oxidation are prevented during heating beforehot-pressing, and a hot-pressed member having very high strength andexcellent corrosion resistance can be produced.

In addition, Patent Literature 3 discloses a hot-pressing method inwhich when a steel sheet coated with Zn or a Zn-based alloy ishot-pressed, an alloy compound such as a Zn—Fe-based compound orZn—Fe—Al-based compound, which prevents corrosion and decarburizationand has a lubricating function, is produced on a surface of the steelsheet during heating before hot-pressing. It is also described that witha hot-pressed member produced by the method, particularly a hot-pressedmember including a steel sheet coated with Zn-50 to 55 mass % Al, theexcellent corrosion preventing effect can be achieved.

Further, Patent Literature 4 discloses a hot-pressing method includingheating a steel sheet provided with a coating, which mainly contains Alor Zn, in an atmosphere having a hydrogen concentration of 6% by volumeor less and a dew point of 10° C. or less at a heating temperature of anAc₃ transformation point or more and 1100° C. or less, and thenhot-pressing the steel sheet, thereby achieving excellent hydrogenembrittlement resistance. In this hot-pressing method, the amounts ofhydrogen and water vapor in the atmosphere during heating are decreasedto decrease the amount of hydrogen entering the steel, therebyattempting to avoid hydrogen embrittlement associated with an increasein strength to over 1000 MPa.

PATENT LITERATURE

[PTL 1] British Patent Publication No. 1490535

[PTL 2] Japanese Patent Publication No. 3931251

[PTL 3] Japanese Patent Publication No. 3663146

[PTL 4] Japanese Unexamined Patent Application Publication No.2006-51543

SUMMARY OF THE INVENTION

However, the hot-pressed members described in Patent Literatures 2 to 4have the problem of hydrogen embrittlement due to the hydrogen entryinto steel with corrosion in the use environment rather than thehydrogen entry into steel during heating before hot-pressing.

Aspects of the present invention provide a hot-pressed member which canbe produced without production of scales, which has excellent coatingadhesion and corrosion resistance after coating, and which can besuppressed from suffering hydrogen entry into steel associated withcorrosion, and also provide a method for producing the same.

As a result of intensive study about the above-described intendedhot-pressed member, the inventors of the present invention obtained thefollowing findings.

i) Hydrogen entry into steel associated with corrosion is suppressed bythe presence of a Ni-diffusion region in a surface layer of a steelsheet which constitutes a member.

ii) Excellent corrosion resistance after coating can be achieved byproviding, on the Ni-diffusion region, an intermetallic compound layercorresponding to a γ phase present in a phase equilibrium diagram of aZn—Ni alloy.

iii) Excellent coating adhesion can be achieved by providing a ZnO layeron the intermetallic compound layer.

Aspects of the present invention have been achieved based on thesefindings and provides a hot-pressed member characterized in that aNi-diffusion region is present in a surface layer of a steel sheetconstituting the member, an intermetallic compound layer correspondingto a γ phase present in a phase equilibrium diagram of a Zn—Ni alloy anda ZnO layer are provided in order on the Ni-diffusion region, and aspontaneous immersion potential indicated in a 0.5 M NaCl aqueousair-saturated solution at 25° C.±5° C. is −600 to −360 mV based on astandard hydrogen electrode.

In accordance with aspects of the present invention, in the hot-pressedmember, preferably, the Ni-diffusion region is present over a range of 1μm or more in the depth direction of the steel sheet, the intermetalliccompound layer is present in an island-like form, and at least onecompound layer selected from a Si-containing compound layer, aTi-containing compound layer, an Al-containing compound layer, and aZr-containing compound layer is provided directly below the ZnO layer.

In accordance with aspects of the present invention, the hot-pressedmember can be produced by heating a Ni-based coated steel sheetincluding a Zn—Ni alloy coating layer, which contains 13% by mass ormore of Ni, on a surface thereof in a temperature region of an Ac₃transformation point to 1200° C., or by heating a Ni-based coated steelsheet including a Zn—Ni alloy coating layer, which contains 10% by massor more and less than 13% by mass of Ni at a coating weight of over 50g/cm² per side of the steel sheet, in a temperature region of an Ac₃transformation point to 1200° C. at an average heating rate of 12°C./second or more; and then hot-pressing the steel sheet. The heating inthe temperature range of the Ac₃ transformation point to 1200° C. ispreferably performed at an average heating rate of 85° C./second ormore.

In addition, as the Ni-based coated steel sheet, it is preferred to usea Ni-based coated steel sheet further including at least one compoundlayer selected from a Si-containing compound layer, a Ti-containingcompound layer, an Al-containing compound layer, and a Zr-containingcompound layer, which is provided on the Zn—Ni alloy coating layer.

As a base steel sheet of the Ni-based coated steel sheet, it ispreferred to use a steel sheet having a composition containing, by % bymass, C: 0.15 to 0.5%, Si: 0.05 to 2.0%, Mn: 0.5 to 3%, P: 0.1% or less,S: 0.05% or less, Al: 0.1% or less, N: 0.01% or less, and the balanceincluding Fe and unavoidable impurities, or a steel sheet furthercontaining, by % by mass, at least one selected from Cr: 0.01 to 1%, Ti:0.2% or less, and B: 0.0005 to 0.08%, and Sb: 0.003 to 0.03% eitheralone or in combination.

According to aspects of the present invention, it is possible to producea hot-pressed member without forming scales, which has excellent coatingadhesion and corrosion resistance after coating and which can besuppressed from suffering hydrogen entry into steel associated withcorrosion. In accordance with aspects of the present invention, thehot-pressed member is preferred as an automobile underbody member andbody structural member having a strength of 980 MPa or more.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a drawing schematically showing a structure of a steel sheetwhich constitutes a hot-pressed member in a cross-sectional directionalong a thickness of the steel sheet.

FIG. 2 is a drawing schematically showing a pressing method used in anexample of the invention.

FIG. 3 is a drawing schematically showing an electrochemical cell usedin an example of the invention.

DETAILED DESCRIPTION 1) Hot-Pressed Member

1-1) Ni-Diffusion Region of Steel Sheet Constituting Member

As described above, the presence of a Ni-diffusion region in a surfacelayer of a steel sheet constituting a member prevents hydrogen entryinto steel associated with corrosion. Although the reason for this isnot necessarily known, it is considered as follows: The hydrogen entryinto a steel sheet due to corrosion is related to oxidation-reductionreaction of Fe rust in a wet environment, and Fe rust is required to bestable rust which is little converted in order to suppress hydrogenentry. A Ni-diffusion region is effective in stabilizing Fe rust, andhydrogen entry into steel associated with corrosion is suppressed by thepresence of the Ni-diffusion region.

However, in order to effectively suppress the hydrogen entry, theNi-diffusion region is preferably present over a range of 1 μm or more,more preferably 2 μm or more, most preferably 3 μm or more, in the depthdirection of the steel sheet constituting the member. Although the upperlimit of the depth is not particularly specified, the effect issaturated at a depth of about 50 μm. The depth of the Ni-diffusionregion can be determined by analysis of a section in the thicknessdirection using EPMA (Electron Probe Micro Analyzer) or analysis in adepth direction using GDS (Glow Discharge Spectroscopy).

As used herein, the term “Ni-diffusion region” represents a region whereNi diffuses into steel from a Ni-based coating layer during heatingbefore hot-pressing is present in a solid-solution state. In addition,since a hot-pressed member of embodiments of the present invention isproduced by hot-pressing a Ni-based coated steel sheet having a Zn—Nialloy layer, the Ni-diffusion region may contain Zn as an impurity, butthe advantages of the present invention are not impaired.

1-2) Intermetallic Compound Layer Corresponding to γ-Phase Present in aPhase Equilibrium Diagram of Zn—Ni Alloy on the Ni-Diffusion Region

An intermetallic compound layer provided on the Ni-diffusion region hasa corrosion potential having a sacrificing anticorrosion effect forsteel and is thus effective for improving corrosion resistance aftercoating. The intermetallic compound layer corresponding to a γ-phasepresent in a phase equilibrium diagram of a Zn—Ni alloy represents alayer composed of an intermetallic compound of any one of Ni₂Zn₁₁,NiZn₃, and Ni₅Zn₂₁. Such an intermetallic compound can be detected bydirect X-ray diffraction of a surface of the member or electron beamdiffraction while observing, with TEM (Transmission ElectronMicroscope), a slice prepared from a section in the thickness directionby FIB (Focused Ion Beam) processing.

In order to achieve the above-described effect of the intermetalliccompound layer, it is desirable to control the abundance of theintermetallic compound layer as described below.

The abundance of the intermetallic compound layer can be measured by anelectrochemical method, i.e., a spontaneous immersion potential in a 0.5M aqueous NaCl air-saturated solution at 25° C.±5° C. on the basis of astandard hydrogen electrode. When the spontaneous immersion potentialbecomes more noble than −360 mV with a small mount of the intermetalliccompound layer, the sacrificing anticorrosion effect for steeldisappears, and the corrosion resistance after coating is degraded. Onthe other hand, when the spontaneous immersion potential becomes lessnoble than −600 mV with a large mount of the intermetallic compoundlayer, the amount of hydrogen generated increases with corrosion, andhydrogen entry may occur even in the presence of the Ni-diffusionregion. Therefore, it is desirable to provide the Ni-diffusion region insuch an abundance that the spontaneous immersion potential in a 0.5 Maqueous NaCl air-saturated solution at 25° C.±5° C. is −600 to −360 mVbased on the standard hydrogen electrode. This abundance is preferablyrealized by allowing the intermetallic compound layer to be present inan island form. In embodiments of the present invention, the island-likeintermetallic compound layer is defined by SEM (Scanning ElectronMicroscopy) observation of a section as follows:

(1) A specimen of 10 mm×10 mm×thickness is cut out from the member,buried in a resin mold, and polished.

(2) The specimen buried and polished in (1) is used and a reflectionelectron composition image is photographed with SEM at a magnificationof 500 times and an acceleration voltage of 5 to 25 kV.

(3) The specimen is photographed in any desired 10 fields of view.

(4) In a photograph, as schematically illustrated in FIG. 1, when theintermetallic compound layer is discontinuously present on a surface ofa steel sheet, a score is “1”, while when the intermetallic compoundlayer is continuously present or not present in a field of view, a scoreis “0”.

(5) When the total score of the 10 photographs is 7 or more, theintermetallic compound layer is determined to be island-like.

1-3) ZnO Layer on Intermetallic Compound Layer Corresponding to γ-PhasePresent in a Phase Equilibrium Diagram of Zn—Ni Alloy

A ZnO layer provided in the outermost layer is excellent not only inadhesion to the intermetallic compound layer but also in adhesion to achemical conversion-treated film formed in pretreatment for coating,thereby significantly increasing coating adhesion. With a thickness of0.1 μm or more, the adhesion to the conversion-treated film becomesatisfactory, while with a thickness of 5 μm or less, the coatingadhesion is not impaired by cohesive failure of the ZnO layer.Therefore, the thickness of the ZnO layer is preferably 0.1 to 5 μm.

Like the intermetallic compound layer, the ZnO layer can be observed byX-ray diffraction or electron beam diffraction through TEM observation,and the thickness thereof can be measured.

The ZnO layer has excellent adhesion to the intermetallic compound layerprovided below the ZnO layer, but the adhesion is further improved byproviding, directly below the ZnO layer, at least one compound layerselected from a Si-containing compound layer, a Ti-containing compoundlayer, an Al-containing compound layer, and a Zr-containing compoundlayer, resulting in more excellent coating adhesion.

2) Production Method

The hot-pressed member of an exemplary embodiment of the presentinvention can be produced by heating the Ni-based coated steel sheetincluding the Zn—Ni alloy coating layer containing 13% by mass or moreof Ni on a surface of the steel sheet in a temperature range of an Ac₃transformation point to 1200° C., and then hot-pressing the steel sheet.

As described above, when the Ni-based coated steel sheet is heated inthe temperature range of the Ac₃ transformation point to 1200° C., Ni inthe coating layer diffuses into the steel sheet, forming theNi-diffusion region. In addition, the intermetallic compound layerdescribed above is formed from the Zn—Ni alloy coating layer provided onthe surface and containing 13% by mass or more of Ni, and at the sametime, Zn partially diffuses to the surface, forming the ZnO layer in theoutermost layer.

Even when the Ni content in the Zn—Ni alloy coating layer is less than13% by mass, the Ni content is 10% by mass or more, and the coatingweight of the Zn—Ni alloy coating layer exceeds 50 g/m² per side of thesteel sheet, so that the hot-pressed member of this embodiment of thepresent invention can be produced by hot-pressing after heating in thetemperature range of the Ac₃ transformation point to 1200° C. at anaverage hating rate of 12° C./second or more.

When the Ni content in the Zn—Ni alloy coating layer is less than 10% bymass or the average heating rate is less than 12° C./second, not onlythe Ni-diffusion region is not sufficiently formed, but also Znevaporation becomes excessively active, thereby failing to form theabove-described intermetallic compound layer. In addition, when thecoating weight of the Zn—Ni alloy coating layer is 50 g/m² or less perside of the steel sheet, the Ni-diffusion region is not sufficientlyformed. Here, the average heating rate of heating in the temperaturerange of the Ac₃ transformation point to 1200° C. is defined as a valueobtained by dividing a temperature difference from room temperature tothe highest ultimate sheet temperature by a time required from roomtemperature to the highest ultimate sheet temperature.

Since the surface of the steel sheet is coated with the Zn—Ni coatinglayer regardless of the Ni content, scales are not produced duringheating before hot-pressing.

The average heating rate of heating in the temperature range of the Ac₃transformation point to 1200° C. is preferably 85° C./second or more.Since the time of retention of the steel sheet at a high temperature isshortened by increasing the heating rate, austenite grains in the steelsheet can be made fine during heating, thereby improving the toughnessof the member after hot-pressing. In addition, Zn evaporation can besignificantly suppressed, and thus corrosion resistance after coatingcan be improved by forming the above-described intermetallic compoundlayer. Further, the excessive formation of the ZnO layer can beprevented, and thus coating adhesion can be stably secured. Such aheating rate can be realized by electric heating or high-frequencyheating.

The Ni-based coating layer of the Ni-based coated steel sheet may be aZn—Ni alloy coating single layer or multiple layers including the Zn—Nialloy coating layer provided on a Ni layer or a Ni-based alloy layer notcontaining Zn. As the Ni-based alloy, an alloy containing Ni and a totalof 20% by mass or less of at least one element selected from Fe, Co, Cr,Mn, Cu, and Mo can be used.

The depth of the Ni-diffusion region and the thickness of the ZnO layercan be adjusted by adjusting the heating conditions (temperature andtime), and the abundance of the intermetallic compound layer can beadjusted by the coating weight of the Ni-based coating. The ZnO layercan be spontaneously formed by usual heating in air or heating in anatmosphere at an oxygen concentration of 0.1% by volume or more.

The Ni-based coating layer described above can be formed by anelectroplating method or the like.

When at least one compound layer selected from a Si-containing compoundlayer, a Ti-containing compound layer, an Al-containing compound layer,and a Zr-containing compound layer is further provided on the Zn—Nialloy coating layer formed on the surface of the steel sheet and isheated in the temperature range of the Ac₃ transformation point to 1200°C., Zn partially passes through the compound layer and diffuses to thesurface, forming the ZnO layer in the outermost layer. Therefore, atleast one compound layer selected from a Si-containing compound layer, aTi-containing compound layer, an Al-containing compound layer, and aZr-containing compound layer can be provided immediately below the ZnO.In this case, when the thickness of the compound layer provided on theZn—Ni alloy layer is 0.1 μm or more, coating adhesion can besufficiently improved, while when the thickness of the compound layer is3.0 μm or less, the Si-containing compound layer is not embrittled, andcoating adhesion is not degraded. Therefore, the thickness is preferably0.1 to 3.0 μm and more preferably 0.4 to 2.0 μm.

Examples which can be applied as a Si-containing compound includesilicone resins, lithium silicate, silicate soda, colloidal silica, asilane coupling agent, and the like. Examples which can be applied as aTi-containing compound include titanates such as lithium titanate,calcium titanate, and the like, a titanium coupling agent containingtitanium alkoxide or a chelate-type titanium compound as a maincomponent, and the like. Examples which can be applied as anAl-containing compound include aluminates such as sodium aluminate,calcium aluminate, and the like, an aluminum coupling agent containingaluminum alkoxide or a chelate-type aluminum compound as a maincomponent, and the like. Examples which can be applied as aZr-containing compound include zirconates such as lithium zirconate,calcium zirconate, and the like, a zirconium coupling agent containingzirconium alkoxide or a chelate-type zirconium compound as a maincomponent, and the like.

The compound layer may be formed on the Zn—Ni alloy coating layer bydepositing on the Zn—Ni alloy coating layer at least one compoundselected from the Si-containing compound, the Ti-containing compound,the Al-containing compound, and the Zr-containing compound and thenheat-drying the deposited compound without water washing. This compoundmay be deposited by any one of a coating method, a dipping method, and aspray method using a roll coater, a squeeze coater, or a die coater. Inthis case, after coating, dipping, or spraying using a squeeze coater orthe like, an air knife method or roll squeeze method may be performedfor adjusting the coating amount and achieving uniformity in appearanceand uniformity in thickness. In addition, heat-drying is preferablyperformed so that the highest ultimate temperature of the steel sheet is40° C. to 200° C., more preferably 50° C. to 160° C.

The compound layer can also be formed on the Zn—Ni alloy coating layerby reactive treatment in which the Ni-based coated steel sheet includingthe Zn—Ni alloy coating layer is dipped in an acid aqueous solutioncontaining at least one cation selected from Si, Ti, Al, and Zr and atleast one anion selected from phosphate ion, hydrofluoric ion, andfluoride ion, and then heat-drying the steel sheet with or without waterwashing.

In order to produce the hot-pressed member having a strength of 980 MPaor more, it is preferred to use, as a base steel sheet of the Ni-basedcoated steel sheet, for example, a steel sheet having a compositioncontaining, by % by mass, C: 0.15 to 0.5%, Si: 0.05 to 2.0%, Mn: 0.5 to3%, P: 0.1% or less, S: 0.05% or less, Al: 0.1% or less, N: 0.01% orless, and the balance including Fe and unavoidable impurities, or asteel sheet further containing, by % by mass, at least one selected fromCr: 0.01 to 1%, Ti: 0.2% or less, and B: 0.0005 to 0.08%, and Sb: 0.003to 0.03% either alone or in combination.

The reason for limiting each of the component elements is describedbelow. Here, “%” representing the content of each component is “% bymass” unless otherwise specified.

C: 0.15 to 0.5%

C is an element which improves strength of steel, and a C content of0.15% or more is required for producing a hot-pressed member having a TSof 980 MPa or more. On the other hand, with a C content exceeding 0.5%,blanking workability of the steel sheet used as a material issignificantly decreased. Therefore, the C content is 0.15% to 0.5%.

Si: 0.05 to 2.0%

Like C, Si is an element which improves strength of steel, and a Sicontent of 0.05% or more is required for producing a hot-pressed memberhaving a TS of 980 MPa or more. On the other hand, with a Si contentexceeding 2.0%, the occurrence of surface defects referred to as “redscales” is significantly increased during hot rolling, the rolling loadis increased, and ductility of the hot-rolled steel sheet deteriorates.Further, with a Si content exceeding 2.0% by mass, when a coating filmmainly containing Zn or Al is formed on the surface of the steel sheetby plating, plating processability may be adversely affected. Therefore,the Si content is 0.05 to 2.0%.

Mn: 0.5 to 3%

Mn is an effective element for improving hardenability by suppressingferrite transformation and is also an effective element for decreasingthe heating temperature before hot-pressing because the Ac₃transformation point is decreased. In order to exhibit such an effect, aMn content of 0.5% or more is required. On the other hand, with a Mncontent exceeding 3%, segregation occurs to decrease homogeneity of thecharacteristics of the steel sheet used as a material and thehot-pressed member. Therefore, the Mn content is 0.5 to 3%.

P: 0.1% or Less

When the P content exceeds 0.1%, segregation occurs to decreasehomogeneity of the characteristics of the steel sheet used as a materialand the hot-pressed member and also decrease toughness. Therefore, the Pcontent is 0.1% or less.

S: 0.05% or Less

When the S content exceeds 0.05%, toughness of the hot-pressed member isdecreased. Therefore, the S content is 0.05% or less.

Al: 0.1% or Less

When the Al content exceeds 0.1%, blanking workability and hardenabilityof the steel sheet used as a material are decreased. Therefore, the Alcontent is 0.1% or less.

N: 0.01% or Less

When the N content exceeds 0.01%, nitride AlN is formed during hotrolling and heating before hot-pressing, and blanking workability andhardenability of the steel sheet used as a material are decreased.Therefore, the N content is 0.01% or less.

The balance includes Fe and unavoidable impurities, but preferably, atleast one selected from Cr: 0.01 to 1%, Ti: 0.2% or less, and B: 0.0005to 0.08%, and Sb: 0.003 to 0.03% are either alone or in combinationadded for the reasons described below.

Cr: 0.01 to 1%

Cr is an effective element for strengthening steel and improvinghardenability. In order to exhibit this effect, the Cr content ispreferably 0.01% or more. On the other hand, when the Cr content exceeds1%, the cost is significantly increased. Therefore, the upper limit ispreferably 1%.

Ti: 0.2% or Less

Ti is an effective element for strengthening steel and improvingtoughness by forming fine grains. Also, Ti forms a nitride in priorityto B described below and is an effective element for exhibiting theeffect of improving hardenability by solid-dissolved B. However, whenthe Ti content exceeds 0.2%, the rolling load during hot rolling isextremely increased, and toughness of the hot-pressed member isdecreased. Therefore, the upper limit is preferably 0.2% or less.

B: 0.0005 to 0.08%

B is an effective element for improving hardenability duringhot-pressing and toughness after the hot-pressing. In order to exhibitthe effect, the B content is preferably 0.0005% or more. On the otherhand, when the B content exceeds 0.08%, the rolling load during hotrolling is extremely increased, and a martensite phase and a bainitephase are produced after hot rolling, thereby causing cracks in thesteel sheet. Therefore, the upper limit is preferably 0.08%.

Sb: 0.003 to 0.03%

Sb has the effect of suppressing the occurrence of a decarburized layerin the surface layer of the steel sheet during the time from heating ofthe steel sheet before hot-pressing to cooling of the steel sheet by aseries of hot-pressing treatments. In order to exhibit the effect, a Sbcontent of 0.003% or more is required. On the other hand, when the Sbcontent exceeds 0.03%, the rolling load is increased, thereby decreasingproductivity. Therefore, the Sb content is 0.003 to 0.03%.

Examples of the heating method before hot-pressing include, but are notlimited to, heating with an electric furnace or gas furnace, flameheating, electric heating, high-frequency heating, inductive heating,and the like.

Examples

Both surfaces of a cold-rolled steel sheet having an Ac₃ transformationpoint of 818° C., a thickness of 1.6 mm, and a composition containing,by % by mass, C: 0.23%, Si: 0.12%, Mn: 1.5%, P: 0.01%, S: 0.01%, Al:0.03%, N: 0.005%, Cr: 0.4%, B: 0.0022%, and the balance including Fe andunavoidable impurities were electroplated in a plating bath containing50 g/L (litter) of sodium sulfate, 100 g/L of nickel sulfatehexahydrate, and 50 g/L of zinc sulfate heptahydrate at pH2 and atemperature of 50° C. with a current density changed from 10 to 50 A/dm²to form Zn—Ni alloy coating layers having different Ni contents andcoating weights shown in Tables 1 and 2. Then, any one of aSi-containing compound, a Ti-containing compound, an Al-containingcompound, and Zr-containing compound shown in Tables 1 and 2 was appliedto each of the steel sheets with some exception and then dried under acondition in which the ultimate temperature was 140° C. to form any oneof a Si-containing compound layer, a Ti-containing compound layer, anAl-containing compound layer, and Zr-containing compound layer having athickness of 0.5 μm. Then, a blank of 200 mm×220 mm collected from eachof the resultant steel sheets as a material was heated at an averageheating rate of 8° C./sec in an air atmosphere in an electric furnacefor 10 minutes at each of the heating temperatures shown in Tables 1 and2. Then, each of the blanks was taken out from the furnace andimmediately drawn by a pressing method schematically shown in FIG. 2 toform hot-pressed member Nos. 1, 4, 7 to 21, 28 to 30, 34, 37, 40, and41. In addition, some of the steel sheets were heated by direct electricheating at an average heating rate of 12° C./sec or 90° C./sec, takenout from the furnace after each of the heating temperatures shown inTables 1 and 2 was attained, and immediately drawn by the same pressingmethod as described above to form hot-pressed member Nos. 2, 3, 5, 6, 22to 27, 31 to 33, 35, 36, 38, and 39. In drawing, the punch width was 70mm, and the processing height was 30 mm. In addition, a sample wascollected from a flat portion of the top of each member, and the depthof the Ni-diffusion region, the thickness of the ZnO layer, and thespontaneous immersion potential, which was an index for the abundance ofthe intermetallic compound layer, were measured by the above-describedmethod. At the same time, the state of the intermetallic compound layerwas confirmed by SEM observation of the section described above. Inaddition, scale resistance, coating adhesion, corrosion resistance aftercoating, and hydrogen entry resistance were examined by the methodsdescribed below.

Scale resistance: evaluated by visually observing a punch non-contactsurface after hot-pressing on the basis of the following criteria:

Circle: Adhesion of no scale

Cross: Adhesion of scales

Coating adhesion: A sample was collected from a flat portion of the topof the member, and a punch non-contact surface was chemicalconversion-treated using PB-SX35 manufactured by Nihon Parkerizing Co.,Ltd. under standard conditions, and then electro-deposition paintGT-10HT gray manufactured by Kansai Paint Co., Ltd. was deposited to athickness of 20 μm under the baking conditions of 170° C. and 20 minutesto form a coated test piece. The conversion-treated andelectro-deposited surface of the thus-formed Lest piece was cross-cut tothe steel base material in a grid-like pattern (10×10 squares, 1 mmspacing) with a cutter knife, and subjected to a cross-cut tape peeltest in which an adhesive tape was applied and peeled. Evaluation wasperformed on the basis of the following criteria, and circle andtriangle marks were regarded as satisfying an object of the presentinvention.

Circle: No peeling

Triangle: Peeling occurred in 1 to 10 squares

Cross: Peeling occurred in 11 or more squares

Corrosion resistance after coating: The conversion-treated andelectro-deposited surface of a test piece prepared by the same method asdescribed above for the coating adhesion was cross-cut with a cutterknife, and then subjected to a corrosion test under corrosion test cycleconditions according to SAE-J2334. The maximum coating blistering widthon one side after 25 cycles was measured and evaluated on the basis ofthe following criteria, and circle and triangle marks were regarded assatisfying an object of the present invention.

Circle: 0 nm≤blistering width<1.5 mm

Triangle: 1.5 nm≤blistering width<3.0 mm

Cross: 3.0 nm≤blistering width

Resistance to hydrogen entry: A sample was collected from a flat portionof the top of the member, and one surface (punch non-contact surface)was mirror-ground to a thickness of 1 mm. Next, the ground surface ofthe sample was Ni-coated and used as a hydrogen detection surface, andthe sample serving as a working electrode and platinum serving as acounter electrode were set in an electrochemical cell schematicallyshown in FIG. 3 to measure the amount of hydrogen entry into steel by anelectrochemical hydrogen permeation method under corrosion of theunground surface at room temperature in air. That is, the hydrogendetection surface side was filled with a 0.1 M aqueous NaOH solution,and a reference electrode (Ag/AgCl) was set through a salt bridge. Inaddition, a 0.5 M NaCl solution was dropped on the unground surface(evaluation surface: punch non-contact surface), followed by corrosionat room temperature in air. The potential on the hydrogen detectionsurface side was set to 0 V vs Ag/AgCl, and the hydrogen permeationcurrent value was continuously measured for 5 days by dropping purewater to the corrosion portion one time per day. The resistance tohydrogen entry with corrosion was evaluated from the maximum currentvalue on the basis of the criteria below. Double circle and circle markswere regarded as satisfying an object of the present Invention. Inaddition, the member on which scales significantly occurred duringhot-pressing was tested after the scales were removed from the surfacesby shot blasting.

Double circle: The maximum current was 1/10 or less of the cold-rolledsteel sheet.

Circle: The maximum current was over 1/10 to ½ or less of thecold-rolled steel sheet.

Cross: The maximum current was over ½ of to the same as the cold-rolledsteel sheet.

The results are shown in Tables 3 and 4. It is found that hot-pressedmember Nos. 1 to 27 and 30 according to aspects of the present inventionare excellent not only in scale resistance, coating adhesion, andcorrosion resistance after coating but also in resistance to hydrogenentry.

TABLE 1 Surface layer structure of steel sheet material Si/Ti/Al/Zr-Heating conditions Surface layer structure of hot-pressed member Zn—Nialloy containing before hot pressing Depth Presence of Thickness Spon-Hot- coating layer compound layer Heating Heating if Ni- State ofSi/Ti/Al/Zr- of taneous pressed Coating Thick- tempera- rate diffusionintermetallic containing ZnO immersion member Ni content weight nessture (° C./ region compound compound layer potential No. (mass %) (g/m2)Type (μm) (° C.) sec) (μm) layer layer (μm) (mV) Remark 1 13 30 No 900 85 Island No 2 −550 Invention Example 2 15 40 No 900 12 3 Island No 2−550 Invention Example 3 14 50 No 900 90 1 Island No 2 −550 InventionExample 4 15 30 Silicone 0.5 950 8 20 Island Yes 2 −500 Invention resinExample 5 13 50 Silicone 0.5 950 12 5 Island Yes 2 −500 Invention resinExample 6 14 4 Silicone 0.5 950 90 2 Island Yes 2 −500 Invention resinExample 7 15 30 Lithium 0.5 950 8 20 Island Yes 2 −500 Inventionsilicate Example 8 15 30 Colloidal 0.5 950 8 20 Island Yes 2 −500Invention silica Example 9 15 30 Silane 0.5 950 8 20 Island Yes 2 −500Invention coupling Example agent 10 16 30 Silicone 0.5 950 8 25 IslandYes 3 −450 Invention resin Example 11 18 30 Silicone 0.5 1000 8 30Island Yes 5 −400 Invention resin Example 12 13 30 No 950 8 10 IslandYes 3 −450 Invention Example 13 13 40 Silicone 0.5 1100 8 30 Island Yes2 −450 Invention resin Example 14 13 50 Silicone 0.5 850 8 20 Island Yes2 −450 Invention resin Example 15 13 20 Silicone 0.5 900 8 20 Island Yes2 −450 Invention resin Example 16 13 30 Lithium 0.5 950 8 20 Island Yes2 −500 Invention titanate Example 17 13 30 Titanium 0.5 950 8 20 IslandYes 2 −500 Invention coupling Example agent 18 13 30 Sodium 0.5 950 8 20Island Yes 2 −500 Invention aluminate Example 19 13 30 Aluminum 0.5 9508 20 Island Yes 2 −500 Invention coupling Example agent 20 13 30 Lithium0.5 950 8 20 Island Yes 2 −500 Invention zirconate Example 21 13 30Zirconium 0.5 950 8 20 Island Yes 2 −500 Invention coupling Exampleagent

TABLE 2 Surface layer structure of material steel sheet Surface layerstructure of Si/Ti/Al/Zr- hot-pressed member Zn—Ni alloy containingHeating conditions Depth State of Presence of Thick- Sponta- Hot-coating layer compound layer before hot pressing of Ni- inter-Si/Ti/Al/Zr- ness neous pressed Ni Coating Thick- Heating Heatingdiffusion metallic containing of ZnO immersion member content weightness temperature rate region compound compound layer potential No. (mass%) (g/m²) Type (μm) (° C.) (° C./sec) (μm) layer layer (μm) (mV) Remark22 10 60 No 950 12 3 Island No 3 −400 Invention Example 23 12 60 No 95012 3 Island No 3 −380 Invention Example 24 12 60 Silicone 0.5 950 12 5Island Yes 2 −500 Invention resin Example 25 10 60 No 950 90 1 Island No3 −400 Invention Example 26 12 60 NO 950 90 1 Island No 3 −380 InventionExample 27 12 60 Silicone 0.5 950 90 2 Island Yes 2 −500 Invention resinExample 28 10 60 No 950 8 5 No No 5 −350 Comparative Example 29 12 60 NO950 8 5 No No 3 −350 Comparative Example 30 12 60 Silicone 0.5 950 12 10Island Yes 2 −500 Invention resin Example 31 10 30 No 950 12 3 Island No3 −400 Comparative Example 32 12 30 No 950 12 3 Island No 3 −380Comparative Example 33 12 30 Silicone 0.5 950 12 5 Island Yes 2 −500Comparative resin Example 34  9 60 No 950 8 2 No No 5 −350 ComparativeExample 35  9 60 No 950 12 1 Island No 3 −350 Comparative Example 36  960 No 950 90 1 Island No 3 −350 Comparative Example 37  9 60 Silicone0.5 950 8 2 No Yes 3 −350 Comparative resin Example 38  9 60 Silicone0.5 950 12 1 Island Yes 3 −350 Comparative resin Example 39  9 60Silicone 0.5 950 90 1 Island Yes 3 −350 Comparative resin Example 40Galvanized steel sheet 950 8 0 No No 5 −700 Comparative Example 41Cold-rolled steel sheet 950 8 0 No No 0 −300 Comparative Example

TABLE 3 Hot- Corrosion Hydrogen pressed Scale Coating resis- entrymember resis- adhe- tance after resis- No. tance sion coating tanceRemarks 1 ◯ Δ ◯ ◯ Invention Example 2 ◯ Δ ◯ ◯ Invention Example 3 ◯ Δ ◯◯ Invention Example 4 ◯ ◯ ◯ ◯ Invention Example 5 ◯ ◯ ◯ ◯ InventionExample 6 ◯ ◯ ◯ ◯ Invention Example 7 ◯ ◯ ◯ ◯ Invention Example 8 ◯ ◯ ◯◯ Invention Example 9 ◯ ◯ ◯ ◯ Invention Example 10 ◯ ◯ ◯ ◯ InventionExample 11 ◯ ◯ ◯ ◯ Invention Example 12 ◯ Δ ◯ ◯ Invention Example 13 ◯ ◯◯ ◯ Invention Example 14 ◯ ◯ ◯ ◯ Invention Example 15 ◯ ◯ ◯ ◯ InventionExample 16 ◯ ◯ ◯ ◯ Invention Example 17 ◯ ◯ ◯ ◯ Invention Example 18 ◯ ◯◯ ◯ Invention Example 19 ◯ ◯ ◯ ◯ Invention Example 20 ◯ ◯ ◯ ◯ InventionExample 21 ◯ ◯ ◯ ◯ Invention Example

TABLE 4 Hot- Corrosion Hydrogen pressed Scale Coating resis- entrymember resis- adhe- tance after resis- No. tance sion coating tanceRemarks 22 ◯ Δ Δ ◯ Invention Example 23 ◯ Δ Δ ◯ Invention Example 24 ◯ ◯Δ ◯ Invention Example 25 ◯ Δ Δ ◯ Invention Example 26 ◯ Δ Δ ◯ InventionExample 27 ◯ ◯ Δ ◯ Invention Example 28 ◯ X X ◯ Comparative Example 29 ◯X X ◯ Comparative Example 30 ◯ ◯ Δ ◯ Invention Example 31 ◯ Δ Δ XComparative Example 32 ◯ Δ Δ X Comparative Example 33 ◯ ◯ Δ XComparative Example 34 ◯ X X X Comparative Example 35 ◯ X X XComparative Example 36 ◯ X X X Comparative Example 37 ◯ X X XComparative Example 38 ◯ X X X Comparative Example 39 ◯ X X XComparative Example 40 ◯ X Δ X Comparative Example 41 X X X XComparative Example

The invention claimed is:
 1. A hot-pressed member comprising a coatedsteel sheet, a base steel sheet of the coated steel sheet havingcomposition containing, by % by mass, C: 0.15 to 0.5%, Si: 0.05 to 2.0%,Mn: 0.5 to 3%, P: 0.1% or less, S: 0.05% or less, Al: 0.1% or less, N:0.01% or less, and the balance including Fe and unavoidable impurities,a Ni-diffusion region which is present in a surface layer of the steelsheet, an intermetallic compound layer which is provided on theNi-diffusion region, and a ZnO layer which is provided on theintermetallic compound layer, the intermetallic compound layercorresponding to a γ phase present in a phase equilibrium diagram of aZn—Ni alloy, wherein a spontaneous immersion potential indicated in a0.5 M NaCl aqueous air-saturated solution at 25° C.±5° C. is −600 to−360 mV based on a standard hydrogen electrode.
 2. The hot-pressedmember according to claim 1, wherein the base steel sheet of the coatedsteel sheet further contains, by % by mass, at least one selected fromCr: 0.01 to 1%, Ti: 0.2% or less, and B: 0.0005 to 0.08%.
 3. Thehot-pressed member according to claim 2, wherein the base steel sheet ofthe coated steel sheet further contains, by % by mass, Sb: 0.003 to0.03%.
 4. The hot-pressed member according to claim 2, wherein theNI-diffusion region is present over a range of 1 m or more in the depthdirection of the steel sheet.
 5. The hot-pressed member according toclaim 2, wherein the intermetallic compound layer is present in anIsland form.
 6. The hot-pressed member according to claim 2, wherein atleast one compound layer selected from a Si-containing compound layer, aTi-containing compound layer, an Al-containing compound layer, and aZr-containing compound layer is provided directly below the ZnO layer.7. The hot-pressed member according to claim 1, wherein the base steelsheet of the coated steel sheet further contains, by % by mass, Sb:0.003 to 0.03%.
 8. The hot-pressed member according to claim 7, whereinthe Ni-diffusion region is present over a range of 1 μm or more in thedepth direction of the steel sheet.
 9. The hot-pressed member accordingto claim 7, wherein the intermetallic compound layer is present in anisland form.
 10. The hot-pressed member according to claim 7, wherein atleast one compound layer selected from a Si-containing compound layer, aTi-containing compound layer, an Al-containing compound layer, and aZr-containing compound layer is provided directly below the ZnO layer.11. The hot-pressed member according to claim 1, wherein theNi-diffusion region is present over a range of 1 μm or more in the depthdirection of the steel sheet.
 12. The hot-pressed member according toclaim 11, wherein the intermetallic compound layer is present in anisland form.
 13. The hot-pressed member according to claim 11, whereinat least one compound layer selected from a Si-containing compoundlayer, a Ti-containing compound layer, an Al-containing compound layer,and a Zr-containing compound layer is provided directly below the ZnOlayer.
 14. The hot-pressed member according to claim 1, wherein theintermetallic compound layer is present in an island form.
 15. Thehot-pressed member according to claim 14, wherein at least one compoundlayer selected from a Si-containing compound layer, a Ti-containingcompound layer, an Al-containing compound layer, and a Zr-containingcompound layer is provided directly below the ZnO layer.
 16. Thehot-pressed member according to claim 1, wherein at least one compoundlayer selected from a Si-containing compound layer, a Ti-containingcompound layer, an Al-containing compound layer, and a Zr-containingcompound layer is provided directly below the ZnO layer.